Methane source fueled combustion devices (e,g, engines), such as lean burn natural gas (NG) engines, are used world-wide for both stationary power generation and mobile applications, inclusive of passenger cars, buses and light and heavy duty trucks. Increased consideration of NG (e.g., CNG) as a fuel supply has been driven by reasons such as an increased availability via fracturing gas extraction technology and also a recognition of potential benefits on the environmental side (and associated assistance in meeting present and anticipated exhaust emission regulations).
For instance, compared to diesel and gasoline engine rivals, NG fuel sourced combustion devices generate fewer pollutants relative to ozone, NOx, and particulate matter (PM). Also, CO2 emissions are reduced because the H/C ratio of NG is about double that of gasoline and diesel fuel.
In the context of the present application, reference to a methane source fueled combustion device is inclusive of, for example, a natural gas (NG) fed combustion chamber used in a stationary power plant and a methane source fueled engine such as an NG fueled engine (e,g., CNG). Methane source fuel supplies include those obtained through oil exploration, coal mining and ocean deposits of methane hydrates.
A further example of a lean burn methane source fueled engine is inclusive of an NG running lean burn mobile vehicle engine, with NG as a sole fuel source for the vehicle engine, as well as the methane source fuel (e.g., NG fuel) as a fuel component of a mixed fuel source supply system to the combustion device such as one that uses NG (e,g., CNG) as a component in a flex-fuel or dual-fuel vehicle (e.g., diesel and NG, or gasoline and NO fuel sources). NG used in vehicles may be classified into CNG and liquefied natural gas (LNG) according to a fuel supply method. The CNG is gas compressed at about 200 atmospheres and is used in a state of being stored in a high-pressure container. The LNG is a cryogenic liquid fuel that is produced by condensing natural gas through cooling of the natural gas to a temperature of −162° C. (−260° F.) while at atmospheric pressure.
CNG relates to natural gas produced out of the ground in a broad sense, but typically refers to combustible gas containing small saturated hydrocarbons as a main ingredient such as methane, ethane and propane with trace levels of butanes and pentanes. CNG is largely classified into oil-field gas produced out of an oil field, coal-field gas produced out of a coal field, and water-soluble gas which is soluble and present in water regardless of occurrence of oil or coal. Each of the coal-field gas and the water-soluble gas contains methane as a main ingredient, and carbon dioxide, oxygen, nitrogen, etc., and is often referred to as dry gas since the gas is not liquefied by pressurization at room temperature. The oil-field gas contains ethane, propane, butane, etc., in addition to the methane, and is often referred to as wet gas since the gas is liquefied by pressurization at room temperature.
Natural gas engines, such as CNG engines, are representative of engines having a fuel source that is predominately methane such that these engines produce emissions that predominately include non-combusted methane-CH4 (e.g., 85%) as well as often other short-chain alkane species (e.g., ethane C2H6 and propane C3H8). Thus, the development of catalysts for high efficiency removal of saturated hydrocarbons, including methane, by oxidation within an exhaust stream is of strategic importance.
Even with catalytic assistance, the removal of methane from the exhaust stream is relatively difficult because the C—H bond must be ruptured. A further feature of methane that makes initial C—H bond cleavage difficult is the highly symmetric shape of methane where all C—H bonds are distributed symmetrically about the central carbon at about 109° resulting in the sticking coefficient of methane being very low on metal or metal oxide surfaces. In the oxidation of higher alkanes, oxidation is generally more easily achieved by the cleavage of C—C bonds. Since the C—H bond is stronger, methane is more difficult to oxidize. Since methane is known to be a powerful greenhouse gas with about 20 times the greenhouse potential of carbon dioxide, there has been investigated the use of noble metals and base metals as catalysts for stimulating the oxidation of methane by cleavage of the C—H bond. Alumina, silica, thoria, and titania supported platinum and palladium catalysts were evaluated in 1983 and 1985 (see C. F. Cullis and B. M. Willatt, Journal of Catalysis, Vol. 83, p. 267, 1983; and V. A. Drozdov, P. G. Tsyrulnikov, V. V. Popovskii, N. N. Bulgakov, E. M. Moron, and T. G. Galeev, Reaction Kinetic Catalysis Letters, Vol. 27, p. 425, 1985). These studies suggested that, under the described conditions, an alumina supported palladium catalyst is the most active, followed by an alumina supported platinum catalyst.
In addition to the treatment of methane, the reduction of non-methane hydrocarbons (NMHCs) from the exhaust of many of these combustion devices (e.g., engines) has also been under consideration and poses challenges. While diesel engines emit very low concentrations of low molecular weight alkalies (e.g., ethane, propane, etc.), these species account for the majority of NMHCs emitted by lean-burn natural gas engines and a fraction equivalent to the natural gas substitution rate for dual-fuel engines. In view of this, more recent investigations have specifically targeted the catalytic oxidation of un-combusted alkanes in order to meet challenging regulatory requirements. For example, the U.S. Environmental Protection Agency (EP) NMHC requirement for heavy-duty on-highway compression-and spark-ignition engines and non-road compression ignition engines is 0.14 g NMHC/bhp·hr (0.19 g NMHC/kW·hr). Also, at least 60% methane conversion is required to meet the stringent European regulations for THC limit values (Tier Euro IV, effective from October 2005).
Accordingly, while methane source fueled engines such as NG engines have the above described advantages (e.g., lower NOx and particulate matter (PM) production); they also have the drawback of the emission of non-combusted methane and, in many instances, non-methane hydrocarbons (NMHCs).
Additional factors presenting challenges, in the emission treatment of methane source fueled combustion devices, such as NG operating engines, include the often relatively low operation temperature (e.g., 400-450° C.) of such devices, and contaminants such as sulfur dioxide (e.g., 1 ppm or more) in, for example, engine exhaust (e.g., SO2 present in the source of NG or introduced to the exhaust stream such as from engine oil or both).
As noted, it has been reported in the literature that oxidation catalysts containing palladium are, under the described conditions, more efficient as compared to platinum-based catalysts in converting methane. However, while palladium-based catalysts have been reported in the prior art to be the most active for methane and NMHCs abatement relative to those studies, they are also known in the art to have serious limitations. For instance, these palladium based catalysts are highly sensitive to sulfur poisoning and their activities toward CH4 oxidation deteriorate very quickly in the presence of SO2 or SO3, and even more quickly when placed in contact with H2S. Since many methane source fueled combustion devices (such as mobile vehicle or stationary engines, as in NG lean burn engines) contain SO2 within the NG itself (e.g., 1-5 ppm) and/or originating from lubricating oils used in many engines, it has been recognized in the art the limitations of using palladium-based catalysts despite their greater efficiency in methane and NMHC's abatement in the exhaust stream. In addition, water vapor is known to be a strong inhibitor on the catalytic activity of methane (and NMHC) oxidation and therefore must also be considered.
Thus, it is understood in the art that the reduction of unburned hydrocarbon emissions from methane source fueled combustion devices such as engines, as in lean-burn NG engines and dual or multi-fuel (e.g., diesel and natural gas) engines and the like, is particularly challenging due to the stability of the predominant short-chain alkane species released (e.g., methane, ethane, and propane). Supported Pd-based oxidation catalysts are generally considered the most active materials for the complete oxidation of low molecular weight alkanes at temperatures typical of lean-burn NG exhaust. However, these catalysts rapidly degrade under realistic exhaust conditions with high water vapor concentrations and traces of sulfur.
The mechanisms associated with sulfur poisoning and regeneration of Pd-based catalysts used in the exhaust of lean burn NG engines have been studied in the prior art. Examples of studies in this regard can be seen in Leprince et al. Regeneration of palladium based catalyst for methane abatement; Paper no.: 210 CIMAC. Congress Kyoto 2004; Hu et al. Sulfur Poisoning and Regeneration of Pd Catalyst under Simulated Emission conditions of Natural Gas Engine 2007-014037 SAE International; and Ottinger et al, Desulfation of Pd-based Oxidation Catalysts for Lean-burn Natural Gas and Dual-fuel Applications 2015-01-0991 SAE International,
As described in the above articles, two primary desulfation strategies have been investigated relative to reactivating poisoned Pd-based catalysts in a lean burn NG engine environment: a) thermal desulfation; and b) reductive desulfation.
Thermal recovery of Pd-based oxidation catalysts has been found to be challenging due to the thermal stability of Pd-sulfur species and the associated minimal sulfur release within suitable (non-damaging) temperature ranges.
Reductive de-sulfation was found to be a better option under the prior art than thermal de-sulfation alone. The above articles describe conversion of the lean burn state in the NG engine with periodic reductive events designed to convert, on a repeating basis, the catalyst exhaust environment over the catalyst from an overall lean air fuel ratio (lambda>1 lean state) to one that is in an overall rich state (lambda<1 rich state). The generation of rich exhaust gas mixtures for engines designed to run under lean conditions is particularly difficult and can have a major negative impact on the drivability and stable operation of the vehicle. In other words, the above described articles all use an overall rich atmosphere to reactivate the catalysts. The same approach of converting a lean burn CNG engine's exhaust from lean to rich in an effort to recover degraded catalyst activity is seen in PCT Publication WO2015167318. One disadvantage of running the Pd-based catalysts under rich conditions is that Pd sinters more rapidly under rich vs. lean exhaust conditions so that re-generation at high temperatures can be detrimental to the overall stability of the catalyst over time. Moreover, an overall rich running state presents a greater likelihood of an increased release of hydrogen sulfide (H2S), which is a more toxic poison relative to Pd, as compared to, for example, sulfur dioxide and other sulfides. An overall lean running engine has a tendency to generate less of the more toxic hydrogen sulfide poison.
Also, in the prior art rich regeneration conditions are considered required since Pd is unique among the noble metals (as in Pt, Pd and Rh) in that elemental S can be incorporated into the bulk of Pd as well as being on the surface. To remove the bulk S, repeated rich-lean cycling is considered required at high temperatures (Ts>700-800° C.). Under rich conditions the elemental S comes to the surface of the Pd/PdO crystallites and then under the lean condition it is readily oxidized to SO2 which is easily desorbed at low temperatures.
A further example of the prior art attempts to offset deactivation of a Pd-based three-way catalyst (“TWC”) provided in a CNG engine system through periodic shifts to a rich (lambda<1) atmosphere, is seen by US 2016/0108833. In US '833 there is described a technique, directed at (general) CNG engine catalyst deterioration avoidance, involving engine control adjustments in the air/fuel ratio from 1.0 (stoichiometric) to 0.99 (rich) when a catalyst is sensed to be in a deteriorated state.
However, as noted above, when dealing with a normally running lean burn engine, shifts from lean to rich states for the purpose of CNG catalyst reactivation, are artificial and hard to achieve by, for example, engine control, or require added complexity and/or lower fuel efficiency. Also, as noted above, the rich running state is considered to have a greater propensity to generate the more toxic hydrogen sulfide as compared to a lean running state.
Thus, the common approaches in the prior art to desulfate a catalyst through either high temperature activation above 600° C. or reductive atmosphere treatment, or a combination of both, has proven to be lacking. For example, the temperature required to regenerate a degraded CNG lean burn catalyst has been found to be beyond the operating temperature range for a lean CNG catalyst and the reducing atmosphere (e.g., by engine control) is hard to achieve. Accordingly, the present invention is directed at addressing such problems in the prior art (e.g., the present invention is directed at avoiding or at least alleviating the aforementioned problems associated with the above described various lean burn combustion devices that are methane source fueled as to result in methane coming in contact with the catalyst in stream).